This invention relates in general to the art of repairing leaking steam generator tubes and more particularly to an improved method of repair of damaged tubes by explosive welding.
In the operation of steam generators wherein heat transfer occurs between fluid passed through the tubes and fluid in contact with the outside of the tubes it sometimes becomes necessary to seal one or more leaks which occur in the tubes, in order to prevent mixing of the fluids. The sealing of the tubes in the past has generally been accomplished by means of expandable sleeves, expansion of the tube into the tubesheet, or by means of welded sealable sleeves or plugs depending on the pressure differential involved.
Ordinarily, where a high pressure steam generator or heat exchanger tube is to be sealed, a cylindrical sleeve is inserted into the bore of the tube, and a weld bead is deposited around the outside circumference of the joint between non defective region of the tube and the sleeve to form a pressure-tight seal. In many cases such tube sealing technique will be satisfactory, however, sometimes it has been found inadequate because fluid can penetrate along the tube sleeve interface and corrode through the weld bead thereby causing leakage. Moreover, in some heat exchangers, it is relatively difficult to seal weld sleeves using conventional equipment because of space limitations.
In the case of a low-pressure heat exchanger tube, the cylindrical sleeve is inserted into the bore of the tube and is either mechanically, hydraulically, or explosively expanded in order to create a mechanical seal between the sleeve and the tube. In most cases these seals tend to leak once the expanded areas have been exposed to several thermal cycles.
Repair of leaking defects in the tubesheet region of tubular heat exchangers has in the past been performed by mechanical, hydraulic or explosive expansion of the tube into the tubesheet at a location above (or below as the case may be) the defective area in order to isolate the leak from the fluid flow outside the tube. This approach is not always effective in stopping leakage as the tube and tubesheet are in mechanical contact; further operation of the steam generator exposes the joint to thermal cycles which loosen the seal. The success of this method depends upon possible corrosion of the tubesheet and/or the absence of debris in the tubesheet-tube crevice, either condition would destroy the leak tightness sought.
All known prior art methods of sealing a defective tube, whether by expansion of tubes to tubesheets, or brazing or welding sleeves to the tube have taken place along the tube at an area away from the defect. Expansion of tubes to tubesheets and sleeves isolate the defective areas of the tube from the fluid flow of either without or within the tube. And in the case of sleeving, the sleeve must be able to withstand the loading forces which acted upon the original tube since the sleeve actually becomes the fluid conduit in the cases where corrosion is in its advanced stages. Sleeves of this type severely limit the fluid flow area within the repaired tube thereby reducing the efficiency of the steam generator.
The present invention avoids the disadvantages of the prior art by employing a method of sealing or patching the cracks in steam generator tubes within the tube sheet areas. This method employs the use of an explosive welding device in order to create a metallurgical bond atop the degraded area of the tube. The sleeve is welded to the inside wall of the tube along the entire length of the sleeve. The thickness size of the sleeve is such that it does not restrict the fluid flow area within the tube as do normal sleeving techniques.